Macroporous p-type silicon Fabry-Perot layers. Fabrication, characterization, and applications in biosensing

We present in this paper that porous silicon can be used as a large surface area matrix as well as the transducer of biomolecular interactions. We report the fabrication of heavily doped p-type porous silicon with pore diameters that can be tuned, depending on the etching condition, from approximately 5 to 1200 nm. The structure and porosity of the matrixes were characterized by scanning force microscopy (SFM) and scanning electron microscopy (SEM), Brunnauer-Emmett-Teller nitrogen adsorption isotherms, and reflectance interference spectroscopy. The thin porous silicon layers are transparent to the visible region of the reflectance spectra due to their high porosity (80-90%) and are smooth enough to produce Fabry-Perot fringe patterns upon white light illumination. Porous silicon matrixes were modified by ozone oxidation, functionalized in the presence of (2-pyridyldithiopropionamidobutyl)dimethylmethoxysilane, reduced to unmask the sulfhydryl functionalities, and coupled to biotin through a disulfide-bond-forming reaction. Such functionalized matrixes display considerable stability against oxidation and corrosion in aqueous media and were used to evaluate the utility of porous silicon in biosensing. The streptavidin-biotin interactions on the surface of porous silicon could be monitored by the changes in the effective optical thickness calculated from the observed shifts in the Fabry-Perot fringe pattern caused by the change in the refractive index of the medium upon protein-ligand binding. Porous silicon thus combines the properties of a mechanically and chemically stable high surface area matrix with the function of an optical transducer and as such may find utility in the fabrication of biosensor devices.